That doesn’t look like anything that should be made directly accessible from the Internet.
I suggest you create a brand new folder just for the ACME Challenges.
Lets call it:
/root/ACMEChallenges/
And use that as the root path in the challenge location section.
For that, just change:
root /root/recipe-app-backend/;
to:
root /root/ACMEChallenges/;
or:
root /var/ACMEChallenges/;
or any other path you like
Then place a test-test file in that folder (with some generic text inside it) and restart nginx.
[note: Linux is case sensitive]